Revert Amplitude's "app" name back to "platform"

This was unintentional.  We only want to rename the Mixpanel telemetry "platform" field to "app."  We want to keep it as "platform" on Amplitude because we want to keep using Amplitude for a bit, and the event fields should be the same.

crates/client/src/amplitude_telemetry.rs

@@ -70,8 +70,7 @@ struct AmplitudeEvent {
     os_name: &'static str,
     os_version: Option<Arc<str>>,
     app_version: Option<Arc<str>>,
-    #[serde(rename = "App")]
-    app: &'static str,
+    platform: &'static str,
     release_channel: Option<&'static str>,
     event_id: usize,
     session_id: u128,
@@ -221,7 +220,7 @@ impl AmplitudeTelemetry {
             user_id: state.metrics_id.clone(),
             device_id: state.device_id.clone(),
             os_name: state.os_name,
-            app: "Zed",
+            platform: "Zed",
             os_version: state.os_version.clone(),
             app_version: state.app_version.clone(),
             release_channel: state.release_channel,